1. Field of the Invention
This invention relates in general to refrigerant expansion devices used in a heat pump. More specifically, this invention relates to an expansion device that has a variable expansion area that is operated by the pressure differential existing between the high pressure and the low pressure sides of a heat pump.
2. Description of the Prior Art
A compression refrigeration system comprises a compressor, a condenser, an expansion device and an evaporator connected in a closed circuit to provide refrigeration. Hot compressed refrigerant vapor from the compressor enters the condenser, where it transfers heat to an external heat exchange medium and condenses. Condensed refrigerant, at a high pressure, flows through the expansion device, where the refrigerant undergoes a pressure drop and at least partially flashes to a vapor. The liquid-vapor mixture then flows through the evaporator where it evaporates and absorbs heat from the external surroundings. The low pressure refrigerant vapor then returns to the compressor to complete the circuit.
Although the expansion device is often of simple construction, its role in the refrigeration system is crucial. Ideally, the expansion device should meter refrigerant in a manner such that refrigerant leaving the evaporator is super-heated by a controlled, relatively small amount. The foregoing is desired to prevent any damaging liquid refrigerant from entering the compressor, and to avoid subjecting the compressor to excessive temperatures from highly super-heated vapor.
The performance of the expansion device plays an important role not only in protecting the compressor, but also in determining the cooling capacity of the refrigeration system. Since the system is a closed circuit, any effect the device has on the low or evaporator side is intimately tied in with the performance of the high or condenser side. Most conventional air conditioning systems incorporating compression refrigeration units of the kind described are designed to have a predetermined cooling capacity at a given ambient temperature. The capacity of the system usually decreases at ambient temperatures above the design point. The decrease in capacity of the system at temperatures above the design point has a direct effect on the requirements related to the type of expansion device.
Among the most commonly used expansion devices are thermostatic expansion valves, capillary tubes and other fixed orifice devices. Thermostatic expansion valves control the flow rate of liquid refrigerant entering the evaporator as a function of the temperature of the refrigerant gas leaving the evaporator. This control is achieved by varying the cross-sectional area through a needle type valve contained within the valve body. The needle is typically joined to a flexible metal bellows or diaphragm which is, in turn, actuated by a non-heat conducting rod connected at its other end to a sealed bellows. The sealed bellows, in turn, is joined to a thermostatic sensing bulb by means of a capillary tube. This bulb provides the feedback to the valve of the temperature of the refrigerant leaving the evaporator and the valve responds by increasing or decreasing the flow of refrigerant through the needle valve according to this temperature. While being highly efficient in their operation and readily responsive to changes in load upon the system to vary the flow of refrigerant to the evaporator, thermostatic expansion valves are also complicated and relatively expensive. Further, in split system type air conditioning systems, wherein the compressor and condenser are located outside at a remote location from the evaporator, the distance of the sensing bulb from the compressor result in less than optimum conditions in such systems.
Capillary tubes are generally used in place of thermostatic expansion valves, particularly in smaller applications, wherein ambient air is almost universally utilized as the condensing medium. Although capillary tubes are relatively inexpensive to manufacture and are simple to install, they have some serious operating limitations, particularly when they are operating at conditions above or below the design point of the system.
As an example, as the outdoor ambient temperature increases there is an increase in the pressure differential across the expansion device and the compressor. At higher pressure ratios the compressor pumps a lower flow rate, however, a capillary type expansion device at such higher pressure ratio passes a higher flow rate. This flow imbalance results in the loss of stored subcooled liquid refrigerant from the high side and loss of superheat and flooding of the low side. These phenomenon greatly reduce system efficiency and performance.
An additional problem is found at relatively low ambient temperatures, wherein the pressure differential across the expansion device is of a relatively small magnitude. Under these conditions the subcooling of the refrigerant entering the expansion device increases as the outdoor ambient temperature falls, leading to more refrigerant being stored in the condenser, which starves the evaporator. As a result, more of the evaporator becomes filled with superheated vapor and the superheat leaving the evaporator increases. Using increased evaporator surface to superheat refrigerant is not putting the surface to its most effective use.
Another known fixed orifice expansion device is the orifice plate. Very simply an orifice plate comprises a thin plate having an expansion orifice extending therethrough. Orifice plates are small and inexpensive, but they are erratic in performance. Hence, such plates are not in wide use.
In an attempt to provide an alternate expansion device, having the economical advantages of the capillary tube, while being small and more efficient in operation, the expansion device described in commonly assigned U.S. Pat. No. 3,642,030 entitled "Refrigerant Throttling Device" and issued on Feb. 15, 1972 in the name of Larry D. Amick was developed. That device comprises a body member having a tubular insert having prescribed length-to-bore diameter ratios, a conical inlet, and a conical exit.
Continuing efforts to develop an economical, efficient and effective fixed orifice expansion device resulted in the development of the expansion device described in commonly assigned U.S. Pat. No. 3,877,248 entitled "Refrigerant Expansion Device" which was issued Mar. 1, 1974 in the name of Fred V. Honnold, Jr. That device comprises a body having an expansion conduit extending therethrough with a flat entrance presenting a sharp edge orifice to incoming refrigerant. The sharp edge entrance orifice effects a major portion of the refrigerant pressure drop at the entrance, so that only a short conduit length is necessary to effect the balance of the pressure drop demanded of the device. The body member is incorporated in a unitary coupling member configured to join refrigerant lines from the condenser to the evaporator.
Commonly assigned U.S. Pat. No. 3,992,898 entitled "Moveable Expansion Valve" which issued Nov. 23, 1976, in the name of Richard J. Duell and John A. Ferrel represents a further refinement of a fixed orifice expansion device. In the device of this patent, the refrigerant metering port is formed in a free floating piston which is mounted within a chamber. When refrigerant flows through this device in one direction, the free floating piston moves to one position wherein the refrigerant flow is through the metering port thereby serving as an expansion device. When refrigerant flows through this device in the opposite direction, the free floating piston moves to a second position wherein refrigerant is allowed to flow through a number of flow channels formed in the outer peripheral surface of the piston to thereby allow substantially unrestricted flow through the device. This arrangement allows such a device to be used, in combination with a second expansion device of the same design, in a heat pump system to allow the desired expansion of the refrigerant through the system flowing in both the cooling and heating directions.
In a cooling only system the expansion device of the '898 patent allows a system to be adjusted as to the amount of refrigerant superheat and other expansion parameters by changing the piston contained within the valve body in the field. The piston usually is changed to match the diameter of the metering port, running the length of the piston, with the requirements of a particular system to optimize performance.
U.S. Pat. No. 4,263,787, issued to the assignee hereof, entitled "Expansion Device with Adjustable Refrigerant Throttling" which issued Apr. 28, 1981 to Albert A. Domingorena relates to an improvement of the device of the '898 patent which allows adjusting the diameter of the metering port without having to break into the refrigeration circuit of the system to change the piston.
Summing up the state of the prior art, thermostatic expansion valves, while being highly efficient in their operation and readily responsive to change in load upon the system to vary the flow of refrigerant to the evaporator, are complicated, expensive, and have drawbacks in certain applications. For this reason they are generally not employed in small applications. As a result, capillary tubes or other fixed orifice expansion devices are generally used in such small applications. Such devices are relatively inexpensive, however, as discussed above, they have operating limitations at both high and low ambient temperatures.
From the foregoing, it is evident that the need exists for a refrigerant expansion device which is inexpensive to manufacture and which is effective in performance over a wide range of operating conditions.
One approach to solving this problem has been to design a refrigerant flow metering device which has a flow metering passage which varies in cross-section in response to changes between the high and low side pressures in the refrigeration system. One such device is described in U.S. Pat. No. 4,341,090 entitled "Variable Orifice Metering" issued on Jul. 27, 1982.
As discussed above in connection with the '898 patent and the '090 patent, it is common practice to use two expansion devices in a heat pump system. One expansion device is dedicated to metering refrigerant in the cooling mode of operation, while the other device allows free bypass flow. Likewise, the other expansion device is dedicated to metering in the heating mode, during which time the cooling expansion device allows free bypass flow. In such a system, the expansion area of the heating expansion device is, as a rule, smaller than the expansion area of the cooling expansion device.
This sizing reflects the operating conditions experienced by the system during the cooling and heating modes of operation. Several examples of problems encountered with a cooling refrigeration system at outdoor temperature extremes were given previously. The problems experienced by a refrigeration system in the heating mode of operation are different, and an appreciation of this facilities an understanding of why an expansion device optimized for the heating mode is desirable.
As an example of the above, in the heating mode of operation, as the outdoor ambient temperature increases there is an increase in the pressure differential across the expansion device and an accompanying increase in flow rate. However, the decreased pressure ratio across the compressor results in an increased flow rate pumped by the compressor. As a result, at times, the evaporator will be starved and the evaporator becomes filled with superheated vapor and the superheat leaving the evaporator increases. Using increased evaporator surface to superheat refrigerant is not putting the surface to its most effective use. The problems associated with this are well known and have been pointed out hereinabove.
An additional problem, in the heating mode, is found at relatively low ambient temperatures, wherein the pressure differential across the system is of a relatively small magnitude but the pressure ratio is high which results in a low flow through the expansion device while the compressor is pumping a low flow rate, which floods the evaporator. As a result, more of the evaporator becomes filled with liquid as it passes from the evaporator to the compressor. The problems associated with this condition are well known and have been pointed out hereinabove.
As a general rule, when a heat pump is operating in the heating mode of operation, it is desirable that the refrigerant flow rate be greater at high evaporator pressures (as a result of high outdoor ambient temperature). While as the evaporator pressure decreases (as a result of decreasing outdoor ambient temperature), refrigerant flow rate is reduced and thus a smaller flow metering area is desired. Such decreased refrigerant flow rate is commensurate with the lower compressor pumping rate at the lower evaporator pressure.
It is accordingly deemed desirable to have a variable area expansion device which is capable of responding to the available system conditions which will allow the device to vary the flow metering passage in accordance with the above noted requirements of a heat pump during the heating mode of operation.